For the prime rib: Preheat the oven to 500 degrees F. Put the butter, horseradish, peppercorns and 2 tablespoons salt in a medium bowl and stir until evenly combined. Slather the seasoned butter evenly over the top and sides of the roast (leave the bottom bare) and put on a rack set in a large roasting pan. Roast for 35 to 40 minutes (5 minutes per pound), then turn the oven off and let the roast sit inside the oven for 1 1/2 hours (do not open the oven door during this time). It should be dark golden brown on the outside and medium rare on the inside.
For the sauce: Meanwhile, whisk the sour cream, horseradish, 2 teaspoons salt and 1 teaspoon pepper together in a medium bowl until well combined. Stir in the chives, cover and refrigerate until ready to serve.
Cut the roast into 1-inch-thick slices and serve with the sauce.
To help plan your meal, decide what time you want to serve the meal and count back about 2 hours--that's when the roast should go into the oven.
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